1. Field of the Invention
The invention relates to a method and apparatus for the quantitative measurement of the production or consumption of gases in power transformers and other high voltage utilities immersed in a transformer oil under normal operational conditions.
2. Description of Prior Art
The existing solution for the identification and diagnosis of faults in power transformers related to their gas production or consumption is based on the so-called DGA method (Diluted Gas Analysis method). This method utilizes the fact that gases, which are either generated directly in the transformer or transported into it's oil filling from the surrounding air, are under standard conditions fully dissolved in the oil filling. To detect whether the given gas is present in the transformer or not and in what concentration, a sample of oil from the oil filling of the given transformer is taken. Gases are then extracted from the oil under vacuum and fed into a DGA analyzer. The analyzer output gives a relatively high accuracy reading of the content of selected gases contained in the sampled oil.
This first step is followed by a diagnostic procedure, which identifies gas sources and the corresponding faults of a transformer. From the absolute values of measured gases and from their proportional representation, the procedure determines the extent of specific faults in the transformer.
However, the practice shows that such a diagnostic method, especially the evaluation of the intensity of the aging of cellulose materials in the transformer, can be highly misleading. The basic and ultimate deficiency of the current DGA diagnostic of transformer faults arises from an improper description of the transportation of gases in the transformer.
Any transformer generally represents an open system where gases, under standard operational conditions (produced/consumed), are diluted in the oil filling of the main tank and transported out of or into the main tank by the oil. The oil steadily flows between the main tank and the conservator (from where gases more or less freely escape into/infiltrate from the atmosphere).
The absolute content of a specific gas in the oil filling of a transformer depends therefore not only on the extent of a specific fault (which produces/consumes the specific gas), but also depends on the intensity of the transport of the specific gas from or into the oil filling of the main tank of a transformer.
The apparent and fundamental deficiency of the current DGA diagnostics is that this method pre-supposes that the transport of gases out of or into a transformer is always more or less constant and does not play any important role there.
This presumption is wrong, because the throughflow of the oil between the main tank and the conservator strongly varies and, in the same way, varies the amount of gases, which escape from and infiltrate into the transformer.
A high or low concentration of a specific gas in the oil filling of the main tank of a transformer is therefore not only caused by either a high or low production or consumption of a specific gas in the transformer. It may also be due to the transport conditions of this gas in or out of the oil filling of the main tank.
This problem can be illustrated by a simple hydraulic analogy. The actual level of the water in a tank depends not only on the inflow but also on the outflow of the water from a tank. With the increase of the water level in this tank, it could be pre-suppose that this increase was induced by the increase of the inflow. This increase however could also be induced by the decrease of the outflow of water, so by only looking at the actual level of the water in the tank, it is not possible to find out what actually caused the change.
Moreover, under the quasi-equilibrium condition (the water level in the tank remains quasi-constant), it is simply impossible to determine the inflow/outflow of the water into/from this tank by a reading of the water level in the tank only (how many litres of water per second flow through the tank?).
These facts illustrate the basic problem of the present DGA diagnostic, because a simple reading of the absolute content of gases in the oil filling of the transformer main tank alone cannot provide us with relevant information about gas sources (faults) or gas sinks (e.g. consumption of O2) in this system.
Theoretically, it is possible to get a very high reading of the specific gas (high level of water in the tank) by the low production of a given gas from a corresponding internal fault (very low inflow of the water in the tank) if a very low escape (very low outflow) of this gas (water) is present from the main tank. On the other hand, the result could be a very low reading of the same gas produced by a relatively big fault if there exists a strong throughflow of the oil between the main tank and the conservator that intensively transports the gas out of the main tank into the surrounding air).
Because the transformer oil always serves as a “porter” of diluted gases, only precise DGA diagnostics of any faults is impossible without the equally precise determination of the oil throughflow between the main tank and the conservator.
The problem is that this direct reading of the oil throughflow is, under standard operational conditions of a transformer, very difficult or impractical. The oil throughflow is here generally controlled by several different variables, namely:                the temperature of the transformer (the change of oil temperature induces the strong dilatation of the oil filling in the main tank),        the temperature difference between the oil filling and the surrounding air, and        typical constants for given transformers corresponding to its specific design for the diameter, length and slope of the tube connecting the upper part of the main tank and the bottom part of the conservator.        
The usual conception that the oil throughflow between the main tank and the conservator is induced only by the dilatation of the oil filling of the main tank is wrong and misleading. Under standard operational conditions there always exists the permanent throughflow of oil between both tanks induced by the thermosiphon effect and its intensity is predominantly affecting the temperature difference between the main tank and conservator.
Any change of the transformer temperature, load, or air temperature thus inevitably and immediately changes the oil throughflow between the transformer tank and the conservator and subsequently the concentration of gases in the oil filling of the transformer main tank, though it's own production/consumption of gases may not change there.
In principle, a relatively negligible fault in the transformer, under given conditions, may be evaluated as a big fault and vice-versa.
Moreover, as already mentioned above, the current DGA represents only so-called qualitative diagnostics. By reading the specific gas in the main tank of a transformer, it can be determined (qualitatively and exactly) that there must exist a specific fault, but because the flow of corresponding gas cannot be measured, the extent of a specific fault cannot be quantified.
The present DGA, based on the quasi-equilibrium saturation of the oil filling of the main tank of a transformer, therefore excludes any standard measuring method of the gas production or consumption in a transformer in the technically required measuring units of m3/s or kg/s.
The current DGA diagnostic then inevitably produces a wide range of more or less anomalous or even wrong conclusions of the status of the monitored transformer.
Generally, the current DGA diagnostic method has at least three fundamental drawbacks:                It does not respect the physical reality of the measured system.        It works only under quasi-equilibrium conditions and is therefore not able to quantify faults in a transformer.        It is not able to discriminate whether the measured growth or decline of the level of the specific gas is caused by either the growth or reduction of the fault, or by the change of boundary conditions (i.e. by the change of the transformer temperature, load etc.).        